Folding knives can be actuated by various methods of externally applied or internally stored energy. Externally powered folding knives function by the user applying force to actuate the deployment of the blade around an axle pin to its full extended position. Internally powered folding knives use energy stored in a spring mechanism, and typically a catch, button, or stud is pressed which allows the blade to rotate about the axle pin and deploy. In addition to these two methods, a more recent development is the hybrid power system which is often known as the “spring assist” design. This design uses an initial externally powered force by the used, typically on a projection on the side of the blade, to start the deployment of the blade. At a certain point in the rotation about the axle pin the spring overcomes an internal stud or projection that provides resistance, and once past that projection the spring force is applied to the blade and fully deploys the blade without any further force applied externally by the user.
In both the internally powered folding knife and the “spring assist” folding knife the blade is powered by a torsion spring. The size and weight of the blade is limited by the power of the torsion spring that can fit in the area near the blade axis point, and the torsion spring itself, and thus its power, is limited by the physical area inside the handle. The geometry of internalizing this torsion spring thus limits the assembly to a defined envelope of blade size, weight, and length compared to the size of the handle.
By moving the spring location to a different location in the handle, and by changing the spring from a torsion type spring to a tension type spring it is possible to increase the limits of blade size, weight, and length without changing the handle size. Connecting the blade to the tension spring allows the function of the mechanism while keeping the tension spring completely internalized and thus protecting this part from damage, abuse, and from the external environment. The use of a tendon also allows a longer distance for the spring to act upon the blade, thus allowing more energy to be applied to the blade overall.